1. Field of the Invention
The invention generally relates to system for monitoring cellular telephone and paging networks and, more particularly, to a portable monitoring system useful for field testing and monitoring network usage and performance parameters in real-time.
2. Description of the Prior Art
A cellular communication system essentially consists of a plurality of cell sites, or base stations, well positioned throughout a geographical region, a Mobile Telephone Switching Office (MTSO), and a plurality of mobile units. Each cell site has a high power antenna system coupled to a control channel transmitter and a control channel receiver, which utilize a plurality of FM duplex channels each comprised of a forward voice channel (FVC) and a reverse voice channel (RVC). The mobile telephone switching office (MTSO) acts as a central coordinating site for the entire cellular network. Each cell in the network connects to the MTSO which, in turn, is connected to a conventional land telephone network.
When a subscriber initiates a call from a mobile unit, a call initiation request is placed on a reverse control channel (RCC). The mobile unit transmits its Mobile Identification Number (MIN), Electronic Serial Number (ESN), and Station Class Mark (SCM), along with the destination telephone number. If a cell cite successfully receives this information, it is forwarded to the MTSO, which may check to see if the subscriber is registered or not, and then assigns the call to a forward and reverse voice channel pair (FVC and RVC) of a public switched telephone line and the conversation commences.
When a subscriber receives a call, the incoming call is received by the MTSO which directs each cell site to transmit on its forward control channel (FCC) a paging message containing the subscriber's mobile identification number (MIN). Each mobile unit constantly monitors the FCC and when its MIN is successfully detected, the mobile unit transmits an acknowledgement signal on the reverse control channel (RCC). Upon a particular cell site receiving the acknowledgement signal, the MTSO directs that site to simultaneously issue a forward voice channel (FVC) and a reverse voice channel (RVC) pair. In this manner, the conversation is carried out on a dedicated channel pair separate from the control channels.
Throughout a typical conversation, as the mobile unit travels throughout the service area, the MTSO coordinates and issues numerous "handoffs" which automatically switch subscribers to different voice channels at different cell sites throughout a service area. Handoff decisions are made by the MTSO when the signal strength on the RVC falls below a predetermined radio signal strength indicator (RSSI) threshold level. Threshold levels are periodically adjusted by the service provider according to such factors as, market growth, system growth, call traffic patterns, and interference.
If a new call for a mobile unit comes in and all of the voice channels of the particular cell site receiving the acknowledgement signal are occupied, then the MTSO directs the cell site to issue a directed retry to the subscriber which directs the subscriber to switch to a different control channel on another cell. Depending on the radio propagation effects and the specific location of the mobile unit, this procedure may or may not result in a successful call. Sometimes the MTSO does not issue a redirect to the subscriber, but rather mistakenly switches the call to an already occupied voice channel. In this instance, a collision occurs generally resulting in both calls being dropped. Other factors, including the performance of the MTSO, the specific channel assignments made, the relative number and location of the cell sites, and the set signal threshold levels may contribute to degrade cellular service resulting in dropped or incompleted calls.
In order to monitor a cellular network's performance, detect fraudulent users, and troubleshoot problems, a service provider must be able to monitor the various control and voice channels. Prior art methods involve hard-wired monitor devices on the MTSO; however, such methods are incapable of monitoring real-time signals or signal strength data at various locations throughout a service area. Often, if a problem is suspected on a particular channel, a field technician is sent into the field with test equipment dedicated to that particular channel. This is inefficient since individual pieces of dedicated test equipment lacks the ability to monitor all channels as well as follow calls from channel to channel as the MTSO routs the call through the system.
In addition to cellular voice technology, mobile paging has come into wide spread use wherein a subscriber carries a portable pager unit on their person which alerts them to an incoming page message. Each page message is sent to a particular paging receiver, which is assigned a unique pager ID, or "cap-code". The cap-code is a seven digit decimal number that identifies a particular pager unit.
A mobile paging system relies on well positioned base station throughout a geographical region. These base stations generally have tall towers which support one or more transmitting antennas typically radiating hundreds of watts of power. A wide-area paging system generally handles messages from many sources. Some paging traffic may originate from the same geographical location as the paging transmitter, while other traffic may be brought to the transmitter via landline, or satellite and microwave links from other geographic areas. Since it is generally the case that the location of a particular user is unknown, most paging systems rely on redundant broadcast of a particular page. In this type of system, the user's page is "sprayed" simultaneously from many different base stations in many different cities so that a user can receive a page wherever they are.
Different paging systems throughout the world, such as, for example POCSAG, GOLAY, and NEC, use several methods to activate a particular pager. For example, some pagers may just beep a certain number of times, others may beep and then display an alphanumeric message. The way a pager responds to a page is determined by a "function code", which is a single decimal digit sent by the paging transmitter. The function code ranges between 0 and 3 for POCSAG and ranges between 1 and 4 for GOLAY. NEC does not specify a function code. Each paging transmission broadcasts one or more pager addresses followed by message block for some or all of the addresses. It is possible for a pager to be paged without any message block. In this case the pager simply beeps with the number of beeps being related to the function code sent by the transmitter.
As paging services proliferate throughout the world, designing more spectrum efficient systems will become paramount. The ability to measure all of the signals in adjacent frequency bands at remote cites throughout a region will become increasingly important to insure acceptable signal-to-noise and interference ratios to design systems which work well in crowded bands.
In addition, many radio systems use genetic analog, digital or subaudible tone signals for control, truncking, and user identification. These generic systems are used, for example, in public, private and military applications. For the proper design of such systems, it is necessary to measure the radiating interference and coverage of the base stations, mobile stations, and portable stations. As personal communications networks (PCNs) proliferate, real-time signal strength measurements will become increasingly important for interference control, capacity control, grade of service, and detection of fraudulent users or transmitters which provide excessive transmitter power or are somehow not properly controlled. For example, future digital radio systems are likely to use dynamic power control for each of the subscriber (mobile or portable) transmitters in order to maintain a desired grade of service for each subscriber. As another example, adaptive antennas might be used to dynamically change where radio energy is radiated based on real-time signal strengths on a wide range of channels. Measurements of signal strength in real time on many frequencies will be a vital element to such dynamic systems. Furthermore, the ability to locate fraudulent users or uncontrolled transmitters will become more important.